1. Field of the Invention
The present invention is directed generally to a heater controller, an image forming apparatus, and a method for on/off control of heaters.
2. Description of the Related Art
Electrophotographic image forming apparatuses typically include a heater, such as a halogen heater, that produces relatively large inrush current as a fixing heater. The fixing heater is desirably controlled so as to lessen flicker because a ratio of electric power used to perform fixation to electric power consumption of the image forming apparatus is typically large.
Flicker measurement is performed by using a flickermeter complying with the standard, IEC 61000-4-15. A flicker value is a quantified value representing degree of visual sensation of flicker annoyance and calculated from a voltage drop, which occurs under actual heater control, relative to a reference voltage or frequency components of voltage waveform. The reference voltage is an average of supply voltages applied to an image forming apparatus over one minute. As the difference between the reference voltage and a dropped voltage after voltage drops due to electric power consumption in the apparatus decreases, the flicker value representing degree of annoyance perception decreases (the degree of flicker is favorably low).
As control schemes for flicker reduction, there are known half-wave control of performing heater-on/off control of a heater according to a heater-on/off pattern in a unit of 10 half-waves, and phase control under which a heater is turned on for only a part of a half-wave and the on-time is made gradually longer. For instance, disclosed in Japanese Patent Application Laid-open No. 2008-191333 is a technique for controlling heaters in this way: when a turn-on duty in a unit of two half-waves of a main heater and a sub heater is greater than 50%, a full turn-on is allocated to one half wave out of the two half waves of each of the heaters such that simultaneous full turn-ons of the main heater and the sub heater does not occur.
However, such a technique as that disclosed in Japanese Patent Application Laid-open No. 2008-191333 for controlling a plurality of heaters is disadvantageous in that a voltage drop can be high when difference between the heaters in electrical power is large or electrical power of the heaters is large. This can increase the difference in voltage from a reference voltage, thereby undesirably increasing flicker value.